1. Field of the Invention
The present invention relates to an area designing apparatus and method for a mobile communication system, the area designing apparatus having a means for calculating a transmission power of a signal transmitted between a base station and each of a plurality of mobile station.
2. Description of the Prior Art
A mobile communication system provides a communication service using a plurality of base stations disposed in the service area thereof. To improve the quality of the communication service, the mobile communication system has an area designing apparatus. The area designing apparatus evaluates the communication quality of a signal transmitted between each of a plurality of mobile stations and one of the base stations. Corresponding to the evaluated result, the communication service provider increases the number of base stations and changes designed radio parameters.
The area designing apparatus randomly places a plurality of communicating mobile stations in a radio zone of a specified base station corresponding to traffic distribution information so as to designate the base station that is radio linked to each of the mobile stations. The area designing apparatus calculates the transmission power of a signal transmitted between the base station and each of mobile stations that are radio linked thereto and evaluates the communication quality of a signal transmitted between the base station and each of the plurality of mobile stations corresponding to the calculated result.
To consider the case that a signal that a specified mobile station transmits interferes with a signal that another mobile station transmits, a plurality of mobile stations are placed. In addition, since the transmission power of a signal transmitted between the base station and each mobile station varies depending on the position of each mobile station, the mobile stations are placed corresponding to traffic distribution information. Thus, the communication quality of the communication service is evaluated in consideration of the traffic distribution.
FIG. 1 is a schematic diagram for explaining the theory of the evaluation of the communication quality of a signal transmitted between each of a plurality of mobile stations and a base station, the evaluation being performed by an area designing apparatus of a conventional mobile communication system. FIG. 1 shows base stations 1 and 2 disposed in the service area of the communication service provider. The base stations 1 and 2 have radio zones 1a and 1b, respectively. Mobile stations 21 to 23 are placed in the radio zone 1a. In reality, there are other base stations and mobile stations along with the base stations 1 and 2 and the mobile stations 21 to 23 shown in FIG. 1.
It is assumed that the mobile communication system uses code division multiple access system (CDMA) that uses one frequency band for each of the up link line and the down link line.
When each of the mobile stations 21 to 23 is in communication and radio linked to the base station 1, the area designing apparatus calculates the transmission power of a signal transmitted from the base station 1 to each of the base stations 21 to 23 and the transmission power of a signal transmitted from each of the mobile stations 21 to 23 to the base station 1.
In addition, corresponding to the calculated result, the area designing apparatus calculates the desired wave power of each desired wave signal and the interference wave power of each interference wave signal transmitted between each of the mobile stations 21 to 23 and the base station 1 and obtains (each desired wave power/each interference wave power). The communication quality is proportional to (each desired wave power/each interference wave power). For example, when the ratio of mobile stations that do not satisfy a predetermined threshold value of (each desired wave power/each interference wave power) is large, the communication provider disposes a new base station and changes the designed radio parameters so as to improve the communication quality.
In this case, to obtain the desired wave power and the interference wave power, the area designing apparatus designates the transmission power of a signal transmitted through each radio line to an constant initial value. In that state, the area designing apparatus randomly and successively selects each radio line and changes the transmission power thereof so that the communication quality of a signal transmitted through the selected radio line satisfies a predetermined target value while the transmission power does not deviate from the range of the predetermined maximum transmission power and the predetermined minimum transmission power. After the area designing apparatus have selected all the radio lines and changed the transmission powers thereof, since a change of the transmission power of a specified radio line causes the interference wave powers of the other radio lines, the communication qualities of the radio lines other than the last selected radio line may deviate from the desired target value even if the transmission powers do not deviate from the range of the maximum transmission power and the minimum transmission power.
For a radio line that has been earlier selected, even if the transmission power of the radio line is in the range between the maximum transmission power and the minimum transmission power, the difference between the communication quality and the predetermined target tends to become large. Thus, to decrease the difference, each radio line is repeatedly selected and the transmission power thereof is repeatedly increased and decreased so as to converge the transmission power and evaluate the communication quality. Thus, the mobile communication system using the code division multiple access method should evaluate the communication quality in such a manner.
In addition, the communication quality of each mobile station placed in an area with a predetermined size is evaluated. As a result, the deterioration ratio which is a ratio of mobile stations that do not satisfy a predetermined level is obtained. The communication qualities at positions of a plurality of mobile stations and the deterioration ratios in a plurality of areas are displayed in colors as visual information on a map.
However, the communication qualities of mobile stations can be evaluated only at random positions thereof corresponding to traffic distribution information. The number of positions at which communication qualities can be evaluated is less in an area where the traffic density is low than another area where the traffic density is high. In addition, the positions at which the traffic qualities are evaluated are randomly designated. Thus, it is desired to improve the area designing apparatus so as to evaluate communication qualities at positions designated at any constant intervals regardless of the traffic density. In addition, the same number of evaluation results of communication qualities is required in each area in order that the communication quality of each mobile station is evaluated in each area having a predetermined size and the ratio of mobile stations that do not satisfy a predetermined level of communication qualities is obtained. However, since the mobile stations are placed at random corresponding to the traffic distribution information, the number of evaluation results of the communication qualities obtained in the individual areas fluctuates. Thus, the statistical reliability of the deterioration ratio calculated in each area varies corresponding to the traffic density.